For energy management purposes, it is important to be able to perform control and monitoring operations (e.g., operating relays and reading electrical power usage meters) of events in an electrical power distribution network from one or more centralized points. In today's technology, this is typically accomplished by using low frequency (3-10 kHz) zero-sequence distribution power line carrier (PLC) communication signals to perform communication with various receivers and transceivers in the substation distribution network. If everything is balanced and under ideal conditions, zero-sequence signals generated within a substation distribution network will not pass through the typical delta-wye substation transformer to the main transmission line. However, due to often unbalanced load conditions and other factors such as transmission line inductances and other reactances, conditions can arise which are favorable to standing wave conditions on the main transmission line at the communication signal frequency which conditions reinforce the amplitude of any unbalanced signals and increase problems from crosstalk via the transmission lines between distribution substations.
Conventional methods used in the prior art to isolate substation transformers from main transmission line PLC circuits operating at high frequency (50-150 kHz) employ components installed directly on the transmission line at correspondingly high voltage, requiring high basic impulse (insulation) level equipment such as parallel-resonant traps tuned to the PLC frequency. Because main transmission line PLC frequencies are comparatively high, the reactive components of such traps are correspondingly small and either require no special disconnect and protection equipment, or that existing for other purposes can be shared. Also, these communication systems typically use only a single conductor rather than all three phase conductors as used in substation distribution lines associated with this invention thus using filter parts for only a single line.
When such conventional prior art signal trapping methods are applied to control distribution PLC signals on substation distribution circuits from reaching the main transmission lines, problems arise. Where the PLC signals are in the neighborhood of 5 kHz, as in one embodiment of the invention, the high voltage and basic impulse level equipment requirements are the same, but the lower PLC frequency used requires reactive components which are correspondingly larger and involve correspondingly lower impedances at the basic power line frequency. Series reactive elements therefore draw correspondingly greater power line frequency currents, thus multiplying the necessary volt-ampere rating and resultant cost of these elements. Moreover, the resulting volt-ampere product requires separate and correspondingly more expensive disconnect and protection devices. Comparatively lower reliability as compared to the prior art solution for high frequency PLC signals also results from the greater component stress requirements of the higher transmission volt-ampere product. The resulting high cost of such a solution as typically used in the prior art ordinarily cannot be as readily justified for the substation distribution PLC function as it can for main transmission line PLC.
If the main transmission line is to be used for communications in addition to distribution communications, the signal trapping using the conventional prior art methods would have to use frequency sensitive bandpass methods so as not to interfere with the main line communication signals. Bandpass attenuation circuits are normally more complicated and thus more expensive and less reliable than either high pass or low pass attenuation circuits.
In view of the above, the present invention attempts to control distribution communications crosstalk between substations by preventing the PLC frequency signals from ever getting to the main transmission line in the first place. This is accomplished by placing a capacitor bank intermediate the substation transformer and any communication equipment on that substation network where the capacitor bank is essentially a shunt to communication frequency signals and yet has a relatively high impedance to the 50 or 60 Hz power frequency. As mentioned supra, if the conditions of no unbalanced feeder phase loading exists, the zero-sequence PLC communication signals will never be transmitted through the substation transformer to the main transmission line.
A further factor for attenuating the PLC frequency signals to or from the transmission line involves connecting an inductance between the common wye-point of the capacitor bank and line neutral to form the shunt element of an L-type attenuation circuit where the series impedance of the substation transformer and the series impedance of the transmission line forms the series element of the L-configured attenuation network. If another substation has a capacitor bank and inductive element configured as just mentioned, this will constitute the other shunt element of a pi attenuator network. However, in both cases the shunt attenuator element of the attenuator network is physically located on the substation distribution network and thus will not in any way interfere with the higher frequency communication signals that may be used only on the main transmission line. This approach eliminates the design problems of cost and reliability inherent in prior art approaches to trapping PLC signals at transmission voltage levels.
In view of the above discussion, it is an object of the present inventive concept to provide an improved attenuator circuit for reducing substation distribution power line carrier communication signal crosstalk via transmission lines between distribution substations.